The present invention relates to wastewater treatment, more specifically solids-liquid separation of activated sludge or concentration of excess sludge, particularly wastewater treatment processes and systems involving solids-liquid separation of activated sludge that can be used for the treatment of organic industrial and domestic wastewaters.
In the water treatment using activated sludge processes, activated sludge must be separated into solids and liquid in order to obtain treated water. A typical method for this purpose involved introducing activated sludge into a clarifier to settle the sludge by gravity and discharging the supernatant as treated water from the clarifier. However, this method required a clarifier having a settling area and a retention time enough to settle activated sludge, resulting in an increased size of the treatment system and a considerable space. When the settling properties of activated sludge were impaired by bulking or other causes, the sludge flowed out from the clarifier to invite deterioration of treated water.
In recent years, a method for separating activated sludge into solids and liquid by membrane separation in place of the use of a clarifier has also been used. In this case, a microfiltration membrane or ultrafiltration membrane is normally used as a solids-liquid separation membrane. However, this method essentially required suction or pressurization by pumping for filtration normally at a pressure of several tens to several hundreds of kPa, which led to a high pumping power and resulted in an increased running cost. Membrane separation had the advantage that a clear effluent free from SS (suspended solids) could be obtained, but also had the disadvantage that the permeation flux was low and that periodic chemical washing was needed to prevent the membrane from contamination.
More recently, an alternative method to the clarifier for solids-liquid separation of activated sludge was proposed, which comprises immersing a filter consisting of a gas-permeable sheet such as a nonwoven fabric in an aeration tank to give a filtrate at a low hydraulic head pressure. A general view of this method is shown in FIG. 1. According to the proposed method, a diffuser tube for aeration 202 and a filter 204 are provided in a biological reaction tank 201, and a diffuser tube 203 for air-washing the filter is placed below the filter. During biological reaction (filtration operation), raw water to be treated is supplied from a raw water feed pipe 205 into biological reaction tank 201 and air or the like are diffused from diffuser tube 202 to perform a biological treatment with activated sludge in the tank, and then the treated mixture of activated sludge and water is passed through filter 204 and the effluent (filtrate) is removed via discharge pipe 206. During this, aeration by diffuser tube 202 generates a cross flow of activated sludge and water mixture (hereinafter also referred to as an activated sludge mixed liquor), including a downflow along the filter surface in biological reaction tank 201 (FIG. 1a). This cross flow forms a dynamic filter layer of activated sludge on the surface of filter 204, and the activated sludge mixed liquor is filtered by the resulting dynamic filter layer and removed via discharge pipe 206. When the filter layer formed on the surface of filter 204 is consolidated to increase the resistance to filtration and decrease the filtrate flow rate, aeration from diffuser tube 202 is stopped and air is diffused from diffuser tube 203 to remove the filter layer on the surface of the filter by air-washing (FIG. 1b). According to this method, a clear filtrate can be obtained by separation with a dynamic filter layer of sludge formed on the surface of the filter. The xe2x80x9cdynamic filter layer of sludgexe2x80x9d here refers to a deposit layer of activated sludge particles formed on the surface of the filter as filtration proceeds. The filter medium of the filter used in this method substantially has a pore size larger than that of activated sludge particles to allow the particles to pass, but a deposit layer of activated sludge particles (a dynamic filter layer of sludge) is formed on the surface of the filter medium under the condition of a low driving pressure for filtration so that this dynamic filter layer prevents activated sludge particles from passing through the medium. Filters commonly used in this method include nonwoven fabrics, woven fabrics, metal nets, etc. What is important in the method using the dynamic filter layer is to evenly and efficiently form a deposit layer of activated sludge particles as a filter layer on the surface of the filter medium with thickness, degree of compaction and other factors suitable for filtering activated sludge in order to reliably prevent the passage of activated sludge particles and to stably obtain treated water with good water quality. In the proposed method, it is defined that the dynamic filter layer is formed by controlling the flow rate of activated sludge flowing along the filter surface at an average of 0.05-0.4 m/s, preferably 0.15-0.25 m/s. In the proposed method, the filtration flux is about 2 m/d and the filtration duration is 2.5 h or more at a flow rate along the filter surface of 0.2 m/s, whereas the filtration flux is initially 4.1 m/d but rapidly deceases to 3.3 m/d after 45 min at a flow rate along the filter surface of 0.03 m/s.
A wastewater treatment system based on an activated sludge process was also proposed wherein a filter is immersed in at least one of a biological reaction tank and a final clarifier and the treated water is drawn from the filter via exits of the filter by the hydraulic head pressure difference from the succeeding tank.
However, these proposed methods had the following disadvantages. In the proposed methods, the flow of the sludge mixed liquor along the filter surface is formed by inducing a flow circulating in a tank by aeration. In these methods, however, the flow rate along the filter surface is not constant so that an even dynamic filter layer of sludge cannot be formed on the filter surface and the sludge readily deposits on the filter surface. Moreover, the water level in the biological reaction tank varies with the water inflow rate and the aeration air flow rate so that the hydraulic head pressure on the filter is not constant and the filtrate flow rate varies rather than remaining at a stable flow rate. If the hydraulic head pressure is unstable and extremely high, the water-permeability of the dynamic filter layer of sludge formed on the surface of the filter deteriorates, which may cause a sharp decline of the filtration flux. As a result, the washing frequency increases and the flux recovery rate after washing decreases. In addition, even minor residual organic contaminants such as BOD (biological oxygen demand) in the raw water entering into the biological reaction tank directly deposit on the filter and a biological slime grows on the surface of the filter to cause a remarkable decrease in filtrate flow rate.
When the filter is immersed in a final clarifier, the following problems occur. In the final clarifier using gravity settling of sludge, the sludge concentration in the clarifier is not even as shown by the fact that a thick sludge deposits at the bottom and the supernatant is collected from the top. Thus, the sludge concentration is uneven at the part where the filter is immersed, with the result that a good dynamic filter layer cannot be formed and a stable effluent cannot be obtained.
We further studied in detail the relation between the filtration flux of the filter and the surface flow rate along the filter in the filtration method using a dynamic filter layer of activated sludge mixed liquor, and found that, when the flow rate along the filter surface is 0.05-0.4 m/s, particularly in a preferred range of 0.15-0.25 m/s, the sludge on the filter surface vigorously flows to make it difficult to form an even dynamic filter layer of sludge with an effective filtration area and the surface of the filter is rapidly blocked with fine sludge flocs to neutralize the effect even with air-washing or water-washing. We further found that a stable dynamic filter layer is formed very rapidly within 5 minutes and a filtration flux of 5 m/d or more can last for 4 hours or more when the surface flow rate is less than 0.05 m/s immediately after washing the filter and that the dynamic filter layer formed on the filter surface can be easily separated only by air-washing under the condition of a surface flow rate of less than 0.05 m/s.
Considering the above problems, we carefully studied to provide a method for more efficient solids-liquid separation of activated sludge mixed liquor, and as a result, we found that the solids-liquid separation of activated sludge can be very efficient by introducing raw water into a biological reaction tank to biologically treat it and then introducing the activated sludge mixed liquor treated in the biological reaction tank into a solids-liquid separation tank containing a water-permeable filter immersed therein to give a filtrate while forming a dynamic filter layer of sludge on the surface of the water-permeable filter. We also found that the dynamic filter layer of sludge can be stably formed on the surface of the water-permeable filter by limiting the moving velocity of the activated sludge mixed liquor along the surface of the water-permeable filter to less than 0.05 m/s in average. We also found that solids-liquid separation more efficiently proceeds when an equalizer (hereinafter also referred to as a flow rectifier) is provided in the solids-liquid separation tank so that the activated sludge mixed liquor passes along and through the surface of the water-permeable filter after it is straightened by the flow rectifier. The present invention was accomplished on the basis of these findings.
Accordingly, the present invention comprises the following aspects.
1. A wastewater treatment process involving solids-liquid separation of the activated sludge mixed liquor obtained after biologically treating raw wastewater, comprising introducing raw water into a biological reaction tank to biologically treat it, then introducing the activated sludge mixed liquor treated in the biological reaction tank into a solids-liquid separation tank containing a water-permeable filter immersed therein and
forming a dynamic filter layer of sludge on the surface of the water-permeable filter to give a filtrate from the water-permeable filter by hydraulic head pressure.
2. A wastewater treatment process involving solids-liquid separation of the activated sludge mixed liquor obtained after biologically treating raw wastewater, comprising introducing raw water into a biological reaction tank to biologically treat it, then introducing the activated sludge mixed liquor treated in the biological reaction tank into a solids-liquid separation tank containing a water-permeable filter immersed therein and forming a dynamic filter layer of sludge on the surface of the water-permeable filter to give a filtrate by sucking the exit side of the water-permeable filter with a pump.
3. The process as defined in 1 or 2 above characterized in that the moving velocity of the activated sludge mixed liquor along the surface of the water-permeable filter is less than 0.05 m/s in average.
4. The process as defined in any one of 1 to 3 above characterized in that a flow rectifier is provided in the solids-liquid separation tank so that the activated sludge mixed liquor passes the surface of the water-permeable filter after it passes the flow rectifier.
5. A wastewater treatment system involving solids-liquid separation of the activated sludge mixed liquor obtained after biologically treating raw wastewater, comprising a biological reaction tank for introducing raw water to biologically treat it, and a solids-liquid separation tank containing a water-permeable filter immersed therein for introducing the activated sludge mixed liquor treated in the biological reaction tank to subject it to solids-liquid separation, characterized in that a dynamic filter layer of sludge is formed on the surface of the water-permeable filter to give a filtrate from the water-permeable filter by hydraulic head pressure.
6. A wastewater treatment system involving solids-liquid separation of the activated sludge mixed liquor obtained after biologically treating raw wastewater, comprising a biological reaction tank for introducing raw water to biologically treat it, and a solids-liquid separation tank containing a water-permeable filter immersed therein for introducing the activated sludge mixed liquor treated in the biological reaction tank to subject it to solids-liquid separation, characterized in that a dynamic filter layer of sludge is formed on the surface of the water-permeable filter to give a filtrate by sucking the exit side of the water-permeable filter with a pump.
7. The system as defined in 5 or 6 above characterized in that a flow rectifier is provided in the solids-liquid separation tank so that the activated sludge mixed liquor passes the surface of the water-permeable filter after it passes the flow rectifier.
According to the present invention, a solids-liquid separation tank is provided at the subsequent stage to a biological reaction tank and a water-permeable filter is immersed in the solids-liquid separation tank, whereby a clear filtrate can be obtained at a lower filtration pressure than used in prior processes.
In the processes of the present invention, the driving pressure used for obtaining a filtrate from the filter can be hydraulic head pressure or suction pressure by pumping. Filtration by hydraulic head pressure has the advantages that any motive power is unnecessary because the driving pressure for filtration uses natural gravity and that a low filtration pressure at which a dynamic filter layer is formed can be easily established, but also has the disadvantage that the filtrate flow rate tends to decrease by the consolidation of the filter layer. In contrast, filtration by suction pressure by pumping has the disadvantages that a motive power is required and that a low filtration pressure at which a dynamic filter layer is formed is difficult to stably keep, but also has the advantage that the filtrate flow rate is less liable to decrease. In the present invention, a more preferred one of these methods can be adopted in view of their disadvantages and advantages.
Suitable water-permeable filters for the present invention include any water-permeable filters known in the prior art such as nonwoven fabrics, filter cloths and metal nets and any of them produce similar effects. The filters can be used in any shape known in the prior art such as planar, cylindrical and hollow shapes, or can be used as a module filter consisting of a bundle of filters.
In the present invention, the average flow rate of the sludge mixed liquor introduced into the solids-liquid separation tank is preferably less than 0.05 m/s along the filter surface in order to stably form a dynamic filter layer on the surface of the water-permeable filter. This allows a good dynamic filter layer to be readily formed on the surface of the filter irrespective of whether the sludge mixed liquor passes downward or upward along the surface of the filter. The average flow rate of the sludge mixed liquor of less than 0.05 m/s along the filter surface reduces the decrease of the filtration flux and stably provides a high flux, whereby the volume of the solids-liquid separation tank can be greatly reduced as compared with prior clarifiers and therefore, a more compact treatment system can be provided. In the present invention, a unidirectional flow of sludge mixed liquor can be formed along the surface of the filter by, for example, removing the sludge mixed liquor treated in the solids-liquid separation tank (concentrated sludge mixed liquor) by a pump or other means on the opposite side to the inlet of the sludge mixed liquor in the solids-liquid separation tank. When the activated sludge mixed liquor treated in the biological treatment tank is introduced from the top of the solids-liquid separation tank into the solids-liquid separation tank, for example, a unidirectional flow of the sludge mixed liquor can be formed along the surface of the filter by removing the concentrated sludge mixed liquor by a pump or other means from the bottom of the solids-liquid separation tank. Thus, the flow rate of the sludge mixed liquor along the filter surface is adjusted by controlling the removal speed of the sludge mixed liquor from the solids-liquid separation tank. The removed concentrated sludge mixed liquor can be returned to the biological reaction tank or sludge thickener or sludge digestion tank or the like or removed as excess sludge.
When the average flow rate of the sludge mixed liquor passing along the surface of the filter is equal to or less than the settling velocity of sludge particles, the sludge mixed liquor is preferably introduced as a downflow with respect to the surface of the filter, i.e. from the top to the bottom of the solids-liquid separation tank. With this arrangement, a good dynamic filter layer of sludge is formed because the influent sludge inevitably passes the surface of the filter even if it settles.
In a more preferred embodiment of the present invention, a flow rectifier is preferably provided in the solids-liquid separation tank so that the activated sludge mixed liquor passes along the surface of the filter after it passes the flow rectifier. With this arrangement, the flow in the solids-liquid separation tank becomes unidirectional and a dynamic filter layer of sludge can be evenly formed on the surface of the filter.
In the system of the present invention, a washer is preferably provided below the filter in the solids-liquid separation tank. The sludge layer formed on the surface of the filter can be readily separated by regularly stopping filtration and washing the filter with this washer. Washing can be either one or both of air-washing and water-washing. During air-washing, the air flow rate is preferably controlled in such a manner that the upflow rate of air bubbles may be at least 0.2 m/s. A perforated tube having larger vent holes than those of prior diffuser tubes is desirably provided as an air-washing tube below the filter module. If such a perforated tube is used, a higher upflow rate can be attained than with diffuser tubes at a comparable aeration volume and ascending air bubbles are also large so that the sludge layer on the surface of the filter can be easily separated. The diameter of vent holes of the perforated tube is preferably 2 mm or more.
In the system of the present invention, sludge enters into the filter module before a dynamic filter layer of sludge is formed on the surface of the filter. Therefore, periodic discharge of sludge is desired to avoid deposition of sludge in the filter module. As a means for this sludge discharge, a sludge discharge pipe penetrating the filter module from the bottom to the inside is preferably connected to introduce waste sludge into the biological reaction tank. The discharging force is preferably gravity fall by hydraulic head pressure, and the hydraulic head pressure for discharging is preferably comparable to the hydraulic head pressure for filtration. However, pumping can be used as a sludge discharging force especially when the driving pressure for filtration is applied by pumping.
In the system of the present invention, the concentrated sludge formed in the solids-liquid separation tank is preferably returned to the biological reaction tank. This enables the BOD load in the biological reaction tank to be properly controlled, leading to a stable biological treatment. The activated sludge mixed liquor is gradually filtered and concentrated as it moves along the surface of the filter. Thus formed concentrated sludge mixed liquor is preferably returned as return sludge to the biological reaction tank. When sludge is introduced downward from the top of the solids-liquid separation tank, the concentrated sludge mixed liquor is preferably returned as return sludge to the biological reaction tank from the bottom of the solids-liquid separation tank.
As described above, the system of the present invention comprises a biological treatment tank and a solids-liquid separation tank, which may consist of a single tank divided by a partition into two chambers in liquid communication with each other through an opening at the bottom of the partition as shown in Example 1 below and FIG. 2, for example, or may consist of two separate tanks connected by a pipe as shown in Example 2 below and FIG. 7, for example.